In the prior art, photo-conductive antennas activated by laser pulses are known. The antenna elements are fabricated from a photo-conductive semiconductor material that becomes conductive when illuminated by a light source, such as a laser, and thus can serve as a metal-like electromagnetic radiator/receiver. When the laser source is turned off, the photo-conductive antenna elements becomes non-conductive. In the non-conductive state the antenna elements cannot transmit or receive electromagnetic waves. In this non-conductive state the antenna elements gives no interference to nearby active antenna elements and are also immune from electromagnetic detection.
To achieve multi-octave frequency coverage with a phased array antenna, multiple layers of planar antenna elements, with each layer dedicated to an octave bandwidth, have been taught to circumvent the problems associated with each antenna element having to cover the entire frequency spectrum. To prevent the formation of extraneous grating lobes at the highest frequency of antenna array operation, the radiating elements must be closely spaced. This restricts the physical size of the antenna array, and the apparatus for optically illuminating the photo-conductive, semiconductor, antenna elements, which in-turn limits its performance at the lower frequencies of operation.
These designs, however, are based on conventional metallic conductor elements and the interaction between the layers proves detrimental to performance. Array designs have also been taught which place portions of metallic conductors on a substrate and interconnect these portions to form a dipole or patch radiator resonant at the frequency of operation. The interconnect function can be performed by an optically controlled switch. Such dipoles or patches, however, are inherently narrow in bandwidth. A departure in operating frequency by more than a few percent requires altering the switch states. Wide instantaneous bandwidth operation is unachievable, and such antenna elements provide interference to other nearby antenna elements in the array.
Thus, there is a need in the art for an antenna that can be used in a phased array without interfering with other nearby antennas in the array, while at the same time being extremely broadband. There is also a need for means to switch a photo-conductive antennas between its conductive and non-conductive states that is small and permits the assembly of an antenna array.